Chronology Building: How to Get a Date Chapter 5 Chronology Building: How to Get a Date

Outline Relative Dating Absolute Dating What Do Dates Mean? How Old Are the Pyramids? The Check, Please

Dating Absolute dates are expressed as specific units of scientific measurement—days, years, centuries, or millennia Relative dates express relationships or comparisons. Stepped pyramid at Saqqara in Egypt is older than Khufu’s pyramid Historic settlement of Williamsburg more recent than the pueblos of Chaco Canyon.

Relative Dating The index fossil concept The law of superposition

Index Fossil Concept The idea that strata containing similar fossil assemblages are of similar age. This concept enables archaeologists to characterize and date strata within sites using distinctive artifact forms that research shows to be diagnostic of a particular period of time.

Law of Superposition The geological principle that in any pile of sedimentary rocks that have not been disturbed by folding or overturning, each bed is older than the layers above and younger than the layers below.

Time Markers Artifact forms that research shows to be diagnostic of a particular period of time

Potsherd Frequencies from Pueblo San Cristobal, New Mexico Depth Corrugated ware Biscuit ware Type I Type II Type III Total 1st foot 57(36.7) 10 (6.5) 2 (1.3) 81 (52.2) 5 (3.2) 155 2nd foot 116(31.3) 17 (4.6) 2 (.01) 230 (62) 6 (1.6) 371 3rd foot 27 (15.3) 2 (1.1) 10 (5.7) 134 (76.1) 3 (1.7) 176 4th foot 28 (21.3) 4 (3) 6 (4.5) 93 (70.9) 0 (0) 131

Nels Nelson’s Excavations at San Cristobal

Potsherd Frequencies from Pueblo San Cristobal, New Mexico Depth Corrugated ware Biscuit ware Type I Type II Type III Total 5th foot 60 (17.3) 15 (4.3) 2 (.01) 268 (77.6) 0 (0) 345 6th foot 75 (18.6) 21 (5.2) 8 (1.9) 297 (73.8) 1(.01) 402 7th foot 53 (23.1) 10 (4.3) 40 (17.5) 126 (55) 229

Potsherd Frequencies from Pueblo San Cristobal, New Mexico Depth Corrugated ware Biscuit ware Type I Type II Type III Total 8th foot 56 (24.6) 2 (.01) 118 (51.9) 51 (22.4) 0 (0) 227 9th foot 93 (45.4) 1 (.01) 107 (52.5) 3 (1.4) 204 10th foot 84 (54.4) 69 (44.8) 154 649 83 364 1,283 15 2,39 4

The Next Step: Seriation A relative dating method. Orders artifacts based on the assumption that one cultural style slowly replaces an earlier style over time. With a master seriation diagram, sites can be dated based on their frequency of several artifact styles.

Seriation Diagram: Nelson’s San Cristobal Potsherd Frequencies

Absolute Dating Helped to shape archaeology into the science it is today

Tree-Ring Dating Also called dendrochronology Developed by Andrew E. Douglass, an astronomer interested in the effects of sunspots on the earth’s climate. Trees in temperate and arctic areas are dormant in the winter and burst into activity in the spring. The result is the formation of concentric growth rings alternating light and dark.

Methodology of Tree-Ring Dating Archaeologist digs up a sample of charcoal or wood with at least 20 rings. The sample is sent to the appropriate lab with contextual data. An analyst will cut or sand the sample down so the rings are easily visible, and the widths are then measured individually. The lab analyst tries to match the sample to the appropriate portion of the regional sequence.

Tree-ring Chronology

Radiocarbon Dating: Archaeology’s Workhorse How It Works: There are three principal isotopes of carbon— 12C, 13C, and 14C. Only one 14C atom exists for every trillion atoms of 12C in living material. 14C is produced in the upper atmosphere, where cosmic radiation creates neutrons that replace one of nitrogen’s protons (14N) to create 14C.

Radiocarbon Dating: How It Works 14C oxidizes and forms carbon dioxide, which is dispersed throughout the atmosphere by stratospheric winds. About 98 percent of all 14C enters the oceans; plants take up the rest through photosynthesis. From plants, it enters herbivores, and then carnivores. All organic life contains radioactive carbon.

Radiocarbon Dating: How It Works As long as an organism is alive, the amount of 14C in it remains in equilibrium with the atmosphere. Once the organism dies, the amount of 14C in its body begins to decrease. After 5,730 years, Cambridge half-life, half of the14C available in a sample, will have converted to 14N. Radiocarbon dating is good for organic remains no more than 45,000 years old.

Radiocarbon Dating: How It Works Example: A sample contains 100 atoms of 14C. After 5,730 years, 50 of these atoms have decayed into 14N. After another 5,730 years, half of the remaining 50 14C atoms have converted to 14N, leaving only 25 14C atoms. After another 5,730 years, this would be halved again to about 12 14C atoms.

Can You Handle Uncertainty? The dating process of 14C is a statistical process The number of beta emissions is not constant over a short period, but the rate averages out over the half-life. Labs measures amount of beta emissions over time and the averages get an age.

Are All Organics Created Equal? Bones and plants are complex. Photosynthetic pathways The chemical process through which plants metabolize carbon. The three pathways discriminate against carbon- 13 differently so that similarly aged plants using different pathways may produce different radiocarbon ages. Archaeologists should try to identify the kind of plant being dated.

The Reservoir Effect Reservoir effect When organisms take in carbon from a source that is depleted of or enriched in 14C relative to the atmosphere. Such samples may return ages that are considerably older or younger than they actually are.

Tree Rings Refine Radiocarbon Dating Hessel de Vries discovered that the amount of 14C is does not remain constant in the atmosphere measuring tree rings. De Vries effects Fluctuations in the calibration curve produced by variations in the atmosphere’s carbon-14 content. Causes radiocarbon dates to calibrate to more than one calendar age.

Accelerator Dating: Taking Radiocarbon to the Limit Problems with the amount of carbon found in the sample limited some sites. Accelerator Mass Spectrometry (AMS) reduced the quantity of datable material required. AMS uses electrostatic tandem accelerator and masspectrometry technology to count the proportion of carbon isotopes in a sample.

Trapped Charge Dating There are three methods for dating trapped charge in artifacts. Thermoluminescence, Optically Stimulated Luminescence and Electron Resonance. Dates extend back at least 300,000 years.

Trapped Charge Dating: Process Over time, background gamma radiation sediment causes some electrons of the atoms of certain minerals to move to a different energy state. Some electrons are “trapped” in atomic imperfections in the minerals’ crystal lattices. Overtime, more electrons are trapped in this way.

Dosimeter A device to measure the amount of gamma radiation emitted by sediments. It is normally buried in a stratum for a year to record the annual dose of radiation. Dosimeters are often a short length of pure copper tubing filled with calcium sulfate.

Trapped Charge Dating

Trapped Charge Dating Techniques Thermoluminescence Used on anything mineral heated to more than 500° C. (ceramics, burnt stone artifacts) http://www.youtube.com/watch?v=T6NxycC7Lco Optically stimulated luminescence Used to date sediments. The age is the time elapsed between the last time a few moments exposure to sunlight reset the clock to zero and the present. Electron spin resonance Can be used to date teeth that are beyond the range of radiocarbon dating.

Argon-Argon Dating Based on the radioactive isotopes decay at certain known rates. Argon-argon dating is a high-precision method for estimating the relative quantities of argon-39 to argon-40 gas. Used to date volcanic ashes between 500,000 and several million years due to their extremely long half-lives. Garniss Curtis used argon-argon dating to estimate the age of Homo erectus in Asia.

What Do Dates Mean? There are many other methods used by archaeologists for dating. We can never date archaeological sites by simple equvalences. Show the dated event contemporaneous with a behavioral event of interest. Old wood problem- old wood was scavenged and reused in a site.

How Old Are the Pyramids? Ages are based on historical documents (i.e. hieroglyphs) Archaeologists Nakhla and Hawass used Radiocarbon dating to date the pyramids. Pieces of carbon from fires were caught and trapped in the mortar. Results revealed the old wood problem for the Old Kingdom dates.

The Pyramids at Giza

The Check, Please Archaeology dating methods are bound by cost and budget. Trapped charge and radiometric dating methods are the most costly. A visit to the site and other background information is often necessary

Summary of Absolute Dating Methods Technique Target material Accuracy Carbon-14 Any organic material; carbon is the most common. To 45,000 BP Thermo-luminescence Ceramics, burnt stone Perhaps back to 300,000 years OSL Quartz, feldspars in eolian sands

Summary of Absolute Dating Methods Technique Target material Accuracy Electronic spin resonance Tooth enamel, burned stone tools, corals, shells 10,000 to 300,000 or more years Argon-argon Volcanic ash 200,000 to several million years

Summary Dating techniques are divided into relative and absolute. Relative dating includes the use of index fossils and seriation. Tree-ring dating (dendrochronology) is useful in relatively small regions. Radiocarbon dating is useful for archaeological sites that are less than 45,000 years old. Accelerator (AMS) technique allows radiocarbon-dating of minute amounts of carbon. Trapped charge dating methods date burned stone tools, ceramics, eolian sediments, tooth enamel. Argon-argon dating is used to date volcanic rock, especially ashes. Dating techniques tell us nothing about cultural activities. Documentary evidence provides dates for historical sites.

Quick Quiz

1. ________ dates are expressed as specific units of scientific measurement, i.e. days, years, centuries, or millennia; _______ dates express relationships or comparisons.

Answer: absolute; relative Absolute dates are expressed as specific units of scientific measurement—days, years, centuries, or millennia; relative dates express relationships or comparisons.

2. The ___ _____ _____ enables archaeologists to characterize and date strata within sites using distinctive artifact forms that research shows to be diagnostic of a particular period of time.

Answer: Index Fossil Concept The Index Fossil Concept enables archaeologists to characterize and date strata within sites using distinctive artifact forms that research shows to be diagnostic of a particular period of time.

The chemical process through which plants metabolize carbon 3. Problems associated with radiocarbon dating include: The chemical process through which plants metabolize carbon The Reservoir effect The De Vries effects B and C only A, B, and C.

Answer: E 3. Problems associated with radiocarbon dating include the chemical process through which plants metabolize carbon, the Reservoir effect, and the De Vries effects.

5. With Trapped Charge Dating, sites can be dated based on their frequency of several artifact styles. True False

Answer: B With a master seriation diagram, sites can be dated based on their frequency of several artifact styles.

The radiocarbon dates on Old Kingdom pyramids were from 100 to 400 years older than documented.

Answer: A The explanation was the old wood problem, where Old Kingdom pharaohs recycled old wood to build their pyramids.